NEW YORK, May 10 (Reuters) - The Trump administration has repeatedly assured Americans that it will not use data from a proposed citizenship question on the 2020 Census to target undocumented immigrants. But more than a quarter of Americans don't believe it, according to a Reuters/Ipsos poll.

That skepticism could have serious implications for the accuracy of the decennial count, according to demographers, activists, local governments and corporations who say it will prompt millions of residents https://reut.rs/2uoFSNK to skip the survey out of fear their participation will result in deportation.

An undercount on the Census could pose lasting problems for communities with high immigrant populations because the survey determines how $900 billion in federal money is allocated https://reut.rs/2IA8P1k, along with how federal and state electoral maps are redrawn https://reut.rs/2G9v8to.

Twenty-six percent of Americans believe the government wants to insert the question about citizenship to "help enforce U.S. immigration laws and detain illegal immigrants." Eight percent believe it is designed to result in an undercount in immigrant communities, according to the poll of more than 2,000 people conducted April 30-May 2.

Thirty percent of respondents, however, view the question as a "standard recordkeeping and reporting procedure," and 21 percent think it will improve Census tallies.

(Full poll results: https://tmsnrt.rs/2VLfPzz) )

A substantial majority of Americans generally support the citizenship question, the poll shows. Sixty-seven percent said they approve of its inclusion, with 41 percent saying they "strongly" approve. Republicans view the attempt to gather data on non-citizens much more favorably than Democrats.

Fear of the question could upend the survey by sharply reducing participation, said Robert Shapiro, who oversaw the Census Bureau as U.S. Under Secretary of Commerce for Economic Affairs during the Clinton administration.

"This is likely to be a very flawed – and perhaps even failed – Census," he said.

Shapiro, now chairman of the consultancy Sonecon LLC, conducted an analysis that found 24.3 million people may avoid the questionnaire because they worry personal data could be shared with law enforcement.

The Commerce Department declined to comment on the poll, but said that "Census responses are safe, secure and protected by federal law."

A spokesman for the Census Bureau did not comment.

Commerce Secretary Wilbur Ross announced in March 2018 that he planned to include the citizenship question to help the Justice Department enforce the Voting Rights Act. It would be the first time the question appears on the survey in 70 years.

But the effort has since been tied up in litigation, with advocates and municipalities arguing that Ross wants to include the citizenship question to scare immigrants out of filling out the Census, thereby costing Democratic regions federal aid and political seats. The Supreme Court heard arguments last month and is widely expected to allow the question.

In an effort to assuage fears among immigrants, government officials, including Ross and Census Bureau Director Steven Dillingham, have repeatedly said they have no plans to share Census respondents' data with immigration authorities.

Legal experts say confidentiality laws clearly prevent any such sharing of Census information on individuals with outside parties, including law enforcement agencies, and any employee who does so could be subject to a fine of as much as $250,000 and up to 5 years in prison.

The last time such data-sharing occurred in a substantive way was during World War II, when the government used Census data to identify Japanese residents and place them in internment camps. Congress later established laws to guard respondents' data privacy.

Those laws are now so well-established that when the Census Bureau received a death threat against President Bill Clinton scrawled across the top of a survey in 2000, officials were unable to tell the Secret Service who had written it, said Shapiro, the former Commerce official.

The author was a prison inmate, which minimized their concerns, but it would have been illegal to share the information in any case, he added.

KNOCKING ON THE DOOR

The Census Bureau's chief scientist, John Abowd, has predicted the inclusion of the question would likely reduce response rates by more than the typical amount. It would also significantly boost the cost of the survey because Census workers will hard to spend more time trying to get people to respond, he has said.

All U.S. residents are required to fill out the survey fully and accurately, making it illegal to dodge it, lie about citizenship status or avoid the question – although that law cannot be enforced with any real consequence because of confidentiality requirements.

Advocates for immigrants and other vulnerable communities say their constituents already have a heightened distrust of the government and are unlikely to hand over information that could be used against them.

President Donald Trump's anti-immigration policies and rhetoric – including separating immigrant children from their parents at the Mexican border, pledging to build a wall there, and implementing a "Muslim ban" – has further stoked those fears, they say.

"It's not going to help having men in suits knocking on your door saying, 'I'm from the government, give me your information,'" Vanita Gupta, a former Justice Department official who now runs the Leadership Conference on Civil and Human Rights, said at a recent event.

Kelly Percival, counsel with The Brennan Center for Justice at New York University Law School, said she hoped U.S. residents feel safe filling out the survey because laws governing data privacy are too strong to be bent on a political whim.